The Sims' designer Will Wright

The Sims' designer Will Wright gives diseased virtual guinea pigs to unwitting players.
"Like almost everything in the game, the guinea pig's function as a disease vector was carefully simulated, Mr. Wright said. For example, the guinea pig only spreads the disease if a Sims player neglects to clean its cage, and only if a player reaches into the cage to pet the software animal and is bitten will he get sick. Someone who has gotten sick sneezes and coughs and will infect other human characters in the game who come within several 'tiles' distance."